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Active optical systems are those in which real-time control over optical wavefronts is employed to optimize system perference in the presence of random distrurbances. Applications include 1) outgoingwave systems for maximizing the power density of a laser beam on a target and 2) received-wave systems for maximizing the angular resolving power of a telescope viewing a distant object through a turbulent atmosphere. In this tutorial-review paper, the basic concepts of active optics systems and their historical evolution are discussed, from early figurecontrol systems with a servo bandwidth of less than 1 Hz to the recently developed high-bandwidth systems for atmospheric compensation with bandwidths of several hundred hertz. A critical comparison of the various approaches to wavefront sensing is then made covering both coherent (laser) and incoherent (white-light)systems. Current techniques for wavefront correction including Bragg cells, segmented mirrors, thin-plate deformable mirrors, monolithic mirrms, and membrane mirrors are described. The performance analysis and optimization of closed loop systems is covered using two basic models. The paper concludes with a review of the design and performance of five current experimental active optical systems, with some comments on future applications.